• kinetics is the study of the factors that affect the speed of a reaction and the mechanism by which a reaction proceeds.. • experimentally it is shown that there are 4 factors that i
Trang 1Chemical Kinetics
Trang 2• kinetics is the study of the factors that affect the speed of a reaction and the mechanism
by which a reaction proceeds
• experimentally it is shown that there are 4
factors that influence the speed of a
reaction:
nature of the reactants,
temperature,
catalysts,
concentration
Kinetics
Trang 3Defining Rate
• rate is how much a quantity changes in a given
period of time
• the speed you drive your car is a rate – the distance your car travels (miles) in a given period of time (1 hour)
so the rate of your car has units of mi/hr
time
distance Speed
Trang 4Defining Reaction Rate
• the rate of a chemical reaction is generally measured in terms of how much the concentration of a reactant
decreases in a given period of time
or product concentration increases
• for reactants, a negative sign is placed in front of the
definition
time
[reactant]
time
[product]
Rate
time
ion
concentrat Rate
Trang 5Reaction Rate Changes Over Time
• as time goes on, the rate of a reaction generally
slows down
because the concentration of the reactants decreases
• at some time the reaction stops, either because the reactants run out or because the system has
reached equilibrium
Trang 6at t = 0
[A] = 8
[B] = 8
[C] = 0
at t = 0 [X] = 8 [Y] = 8 [Z] = 0
at t = 16
[A] = 4
[B] = 4
[C] = 4
at t = 16 [X] = 7 [Y] = 7 [Z] = 1
1 6 0 0.25
0
4 Rate
t t
C C
t
C Rate
1 2
1 2
1 6 0 0.0625
0
1 Rate
t t
Z Z
t
Z Rate
1 2
1 2
Trang 7
1 6 0 0.125
4
6 Rate
t t
C C
t
C Rate
1 2
1 2
1 6 0 0.0625
7
6 Rate
t t
X X
t
X Rate
1 2
1 2
at t = 16
[A] = 4
[B] = 4
[C] = 4
at t = 16 [X] = 7 [Y] = 7 [Z] = 1
at t = 32
[A] = 2
[B] = 2
[C] = 6
at t = 32 [X] = 6 [Y] = 6 [Z] = 2
Trang 8
1 6 0 0.0625
6
5 Rate
t t
X X
t
X Rate
1 2
1 2
1 6 0 0.125
2
0 Rate
t t
A A
t
A Rate
1 2
1 2
at t = 32
[A] = 2
[B] = 2
[C] = 6
at t = 32 [X] = 6 [Y] = 6 [Z] = 2
at t = 48
[A] = 0
[B] = 0
[C] = 8
at t = 48 [X] = 5 [Y] = 5 [Z] = 3
Trang 9Hypothetical Reaction
Time
(sec)
Number Red
Number Blue
in this reaction, one molecule of Red turns into one molecule of Blue
the number of molecules will always total 100
the rate of the reaction can
be measured as the speed of loss of Red molecules
over time, or the speed of gain of Blue molecules
Trang 10Hypothetical Reaction
Red Blue
Concentration vs Time for Red -> Blue
100
84
71
59
50
42
35
30
25
21
18
0
16
29
41
50
58
65
70
82
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
Number Red Number Blue
Trang 11Hypothetical Reaction
Red Blue
Rate of Reaction Red -> Blue
5, 3.2
10, 2.6
15, 2.4
20, 1.8
25, 1.6
30, 1.4
35, 1 40, 1
45, 0.8
50, 0.6
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
,
]/
Trang 12Reaction Rate and Stoichiometry
• in most reactions, the coefficients of the balanced
equation are not all the same
H2 (g) + I2 (g) 2 HI(g)
• for these reactions, the change in the number of
molecules of one substance is a multiple of the change in the number of molecules of another
for the above reaction, for every 1 mole of H2 used, 1 mole of I2 will also be used and 2 moles of HI made
therefore the rate of change will be different
• in order to be consistent, the change in the concentration
of each substance is multiplied by 1/coefficient
t
HI]
[ 2
1 t
] I [ t
] H
[ Rate 2 2
Trang 13Average Rate
• the average rate is the change in measured
concentrations in any particular time period
linear approximation of a curve
• the larger the time interval, the more the average rate deviates from the instantaneous rate
Trang 14Hypothetical Reaction Red Blue
Avg Rate Avg Rate Avg Rate
Time
(sec)
Number Red
Number Blue
(5 sec intervals)
(10 sec intervals)
(25 sec intervals)
Trang 15H2 I2
HI
Avg Rate, M/s Avg Rate, M/s Time (s) [H2], M [HI], M -[H2]/t 1/2 [HI]/t
0.000 1.000
10.000 0.819
20.000 0.670
30.000 0.549
40.000 0.449
50.000 0.368
60.000 0.301
70.000 0.247
80.000 0.202
90.000 0.165
Avg Rate, M/s Avg Rate, M/s Time (s) [H2], M [HI], M -[H2]/t 1/2 [HI]/t
0.000 1.000 0.000
10.000 0.819 0.362
20.000 0.670 0.660
30.000 0.549 0.902
40.000 0.449 1.102
50.000 0.368 1.264
60.000 0.301 1.398
70.000 0.247 1.506
80.000 0.202 1.596
90.000 0.165 1.670
Avg Rate, M/s Time (s) [H2], M [HI], M -[H2]/t
0.000 1.000 0.000
10.000 0.819 0.362 0.0181
20.000 0.670 0.660 0.0149
30.000 0.549 0.902 0.0121
40.000 0.449 1.102 0.0100
50.000 0.368 1.264 0.0081
60.000 0.301 1.398 0.0067
70.000 0.247 1.506 0.0054
80.000 0.202 1.596 0.0045
90.000 0.165 1.670 0.0037
Trang 16Concentration vs Time for H 2 + I 2 > 2HI
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
1.600
1.800
2.000
0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 80.000 90.000 100.000
[H2], M [HI], M
average rate in a given
the line connecting the
of the line for [HI]
the average rate for the first 10 s is 0.0181 M/s
Trang 17Instantaneous Rate
• the instantaneous rate is the change in
concentration at any one particular time
slope at one point of a curve
• determined by taking the slope of a line tangent
to the curve at that particular point
first derivative of the function
for you calculus fans
Trang 18H 2 (g) + I 2 (g) 2 HI (g) Using [H
2], the instantaneous rate at
50 s is:
s
M
0.0070 Rate
s 40
M 28 0 Rate
Using [HI], the instantaneous rate at
50 s is:
s
M
0.0070 Rate
s 40
M 56 0 2
1 Rate
Trang 19Ex 13.1 - For the reaction given, the [I] changes from 1.000 M to 0.868 M in the first 10 s Calculate the
average rate in the first 10 s and the [H+]
H2O2 (aq) + 3 I(aq) + 2 H+(aq) I3(aq) + 2 H2O(l)
Solve the equation
for the Rate (in
terms of the change
in concentration of
the Given quantity)
Solve the equation
of the Rate (in terms
of the change in the
concentration for the
quantity to Find) for
s
M 3
-10 4.40
Rate
s 10
M 000
1 M 868 0 3
1 t
] I [ 3
1 Rate
] H [
Rate
2 t
] H [
t
] H [ 2
1 Rate